Automatic postural adjustments responsible for human balance will be studied in young and old normal adults, in patients with disorders of posture control, and in elderly people with undiagnosed falling problems. The goal of these studies are 1) to define, quantitatively, the normal repertoire of postural movement strategies in standing humans for different speeds and amplitudes, 2) to determine the effects of peripheral sensory versus central cerebellar disorders on selection and parameterization of postural strategies and approach, and 3) to determine whether falling in the elderly can be related to specific peripheral or central nervous system degeneration. Subjects will stand on a movable platform system. Postural adjustments to a variety of different movement perturbations will be described by analyzing the EMG activity of selected leg and lower trunk muscles and by recording surface forces and motions of the ankle, knee, hip and neck angles. Experiments are designed to test the hypothesis that the CNS preprograms a discrete set of automatic postural strategies, each which is normally triggered within bounded stimulus conditions and that these boundaries can change with neural pathology causing postural instability in the elderly. We predict that peripheral sensory system loss will primarily affect the stimulus boundaries from which intact postural responses are selected whereas central (cerebellar) damage will affect both the internal structure of the postural response and the ability to use prior experience to 'fine tune' the response. Initially, select groups of young and old patients with well-defined peripheral vestibular deficits, peripheral neuropathy and cerebellar deficits will be studied. Results will be used to determine if specific components of postural dyscontrol could be used to identify specific peripheral and central nervous system degenerations in elderly people with well-documented stability problems.